The Northern and Southern Scandes — structural differences revealed by an analysis of gravity anomalies, the geoid and regional isostasy

We investigate the Scandes mountain range by analysing the gravity field, the geoid heights and the degree of isostatic compensation of the lithosphere. Topographically, the Scandes mountain range can be divided in the Northern and Southern Scandes. Comparisons between the present topographic expression and the gravity field and the geoid show that the axis of highest elevation in the Northern Scandes is shifted eastwards compared to the minimum of the Bouguer anomaly, while the two coincide perfectly in the Southern Scandes. Geoid heights reduced by the effect of topographic masses show a large-scale minimum in the Northern Scandes, but no anomaly in the Southern Scandes.Regional, flexural isostatic calculations yield a flexural rigidity of D = 10²³ Nm for the lithosphere of the Southern Scandes and the isostatic gravity and geoid residuals point to additional isostatic support by low-density rocks below the Moho. On the other side, for the lithosphere in the Northern Scandes no significant flexural rigidity can be resolved. Here, the Bouguer anomaly is best modelled with a small flexural rigidity, indicating nearly Airy isostatic behaviour. Local subsurface loading and horizontal tectonic forces overprint the isostatic compensations and increase the tectonic complexity of the Northern Scandes. These distinctive features of the Scandes cannot be explained by currently existing models of the present and Neogene uplift and the isostatic mechanism of the Scandes.     

EGMlab, a scientific software for determining the gravity and gradient components from global geopotential models

Nowadays, Global Geopotential Models (GGMs) are used as a routine stage in the procedures to compute a gravimetric geoid. The GGMs based geoidal height also can be used for GPS/levelling and navigation purposes in developing countries which do not have accurate gravimetric geoid models. Also, the GGM based gravity anomaly including the digital elevation model can be used in evaluation and outlier detections schemes of the ground gravity anomaly data. Further, the deflection of vertical and gravity gradients components from the GGMs can be used for different geodetic and geophysical interpretation purposes. However, still a complete and user-friendly software package is not available for universities and geosciences communities. In this article, first we review the procedure for determination of the basic gravity field and gradient components from the GGMs, then general MATLAB based software is presented and applied as a sample case study for determination of gravity components based on the most recent EIGEN-GL04C GRACE model in Sweden. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Late Pleistocene Afton Lodge Clay Formation,Ayrshire, Scotland: evidence for Early to Middle Devensian climatic changes and Late Devensian onshore ice flow and rafting from the Firth of Clyde

An investigation into the late Pleistocene sediments exposed at Afton Lodge has helped to clarify the glacial history of western central Scotland. The sequence includes several allochthonous bodies of ‘shelly clay’ (Afton Lodge Clay Formation) associated with Late Devensian (Weichselian) age diamict. The shelly clay contains abundant marine macro- and microfauna, as well as palynomorphs consistent with its deposition within a shallow marine to estuarine environment. Faunal changes within the main body of marine clay record at least one, millennial-scale cycle of Arctic-Boreal, to Boreal, and back to Arctic-Boreal climatic conditions. A radiocarbon date of over 41 ka ¹⁴C BP obtained from the foraminifera indicates that the marine clays are older than the surrounding till. Afton Lodge is thus one of a suite of ‘high-level’ shelly clay occurrences around the Scottish coasts that are now considered to be glacially transported. Together with closely associated ‘shelly tills’, the rafts were emplaced during an early phase of the last glaciation by ice flowing from the western Grampian Highlands of Scotland through the topographically-confined Firth of Clyde basin. The blocks of marine sediment were detached subglacially, unfrozen, and carried at least 10 km by ice that splayed out onshore against reversed slopes favouring raft emplacement and the creation of closely associated ribbed moraine. Transport of the rafts was facilitated by water-lubricated décollement surfaces and their accretion was accompanied by dewatering. The shelly tills were formed mainly by the attenuation and crushing of rafts of shelly clay during their transport within the subglacial deforming bed.     

Delimitation of ground failure zones due to land subsidence using gravity data and finite element modeling in the Querétaro valley, México

Sedimentary basins of arid and semiarid zones are often subject to regimes of intense ground-water withdrawal as it is normally the only source of water for development of communities. An associated phenomenon is land subsidence, which can develop to ground failures, and consequently, damage to infrastructure. Aquifer deformation can be analyzed using a stress–strain or a flux–force approach depending on the aquifer material (compact or loose) and on whether the water withdrawal forms a predominant flow direction toward a cone of depression. Geometry of the aquifer system also plays an important role as uneven thickness induces differential compaction and hence, tensional and shear stresses on the ground mass. In this work we present a stress–strain approach to analyze subsidence for an unconfined aquifer of varying thickness; this is done in two steps, namely when the aquifer is in equilibrium, and when it is totally depleted. Using this scheme in a region where ground failure is evident, a portion of the aquifer system of the Querétaro valley is analyzed. The geometry of the hydrologic basement is first modeled using gravity measurements properly correlated with wells and field data. Then a stress analysis is implemented using the finite element method in order to identify probable zones of ground weakness, which are calibrated with known ground failures. The results indicate that both, tensional and shear stress are present, which induce ground failure in the form of surface faults.